Laying the Foundation: Designing the LifeKart Around Its Battery
When building an electric vehicle from the ground up, you eventually have to decide what you are building the rest of the machine around. For the LifeKart, that decision was easy: the battery pack.
The battery pack will be the largest, heaviest, and least configurable component of the entire build. Because we are using large-format Lithium Iron Phosphate (LiFePO4) cells, we don't have the same flexibility in layout or connection sequences that you get with smaller cylindrical lithium-ion cells. So, instead of trying to find a place to stuff the battery later, we are designing the LifeKart chassis and component layout specifically to accommodate the pack. This ensures the kart is balanced, safe, and structurally sound from the start.
Calculating Voltage: The Need for 32 Cells
The LifeKart is being designed to run on a 96-volt system. But how did we land on a specific cell count? It starts with simple math using the nominal voltage of our cells.
First, a quick refresher: When calculating pack voltage, you must always use the nominal voltage specific to your cell chemistry. For our LiFePO4 cells, the nominal voltage is approximately 3.2 volts.
If we only did the basic math, it would look like this: 96v / 3.2v = 30 cells (Series)
At first glance, a 30-cell pack gets us to 96 volts. Problem solved, right? Not quite.
We have to think about the discharge curve. What happens when the pack is less than half charged, and the voltage sags to, say, 3.0 volts per cell? We run the same equation, but in reverse: 30 cells x 3.0v = 90v
Now we are 6 volts below our target. While running at 90 volts won't break anything, our kilowatt output—the "punch" you feel when you hit the throttle—will be noticeably reduced at lower states of charge. To maintain that performance throughout the entire discharge cycle, we need to compensate by adding a few more cells in series.
If we calculate based on the lower voltage threshold (3.0v) instead of the nominal voltage, we get a different result:
96v / 3.0v = 32 cells (Series)**
By adding just two extra cells to the pack, we drastically improve the usable power band. With 32 cells, we now hit our target 96 volts at 50% charge or greater, rather than 70% or greater.
- 30 cells: 96v available at 70%+ state of charge
- 32 cells: 96v available at 50%+ state of charge
Note: Cell voltages vary slightly across the market. Some LiFePO4 cells are happy up to 3.6v, while others can drain down to 2.4v without issue. The numbers above represent a healthy average for standard performance cells.
Bigger is Better? (And How We Solved the Space Issue)
So, we’ve settled on a 32S1P configuration. The math works, and the power band looks great. However, we now face a physical challenge.
As you can see in the photo below, these LiFePO4 cells are massive compared to the typical 18650 or 21700 cylindrical cells used in most modern EVs. [Image: https://placehold.co/1200x800]
If we simply lined up 32 of these bricks in a row, we would end up with a battery pack longer than the kart itself. We could extend the frame and mount it behind the seat, but that would ruin the weight distribution and make the kart unnecessarily long and clumsy.
Our solution? We cut it in half.
That's right—the final LifeKart battery pack will actually be two separate modules wired in series. Each module will house 16 cells, connected with heavy-duty custom plugs to link them together. This allows us to mount one module on either side of the driver, keeping the weight low and centered for optimal handling.
In the crude visual below, you can see how splitting the pack transforms the layout of the kart. [Image: Placeholder for visual description]
By splitting the pack, we keep the chassis compact and the center of gravity exactly where it should be. In future posts, we’ll dive into the nitty-gritty details of wiring, assembly, and the custom bus bars required to bring these two halves to life.